1. Field of the Invention
The invention relates to an image transmitting method and, more particularly, to an image transmitting method whereby a variable length coding is performed on image information and the image information which has been variable length coded and is compressed is transmitted together with information other than the image information on a data block unit basis of a predetermined data amount.
2. Related Background Art
In recent years, in the field of digital transmission of a color image, a high efficiency coding technique for information has been advanced and high compression is being realized.
In association with this technique, a good image can be transmitted and received through a transmission line even at a low data rate. On the other hand, however, a degree of influence which is exerted on the image by an error of one word on the transmission line also increases. Therefore, it is necessary to take a countermeasure for a code error on the transmission line by an error detection code and an error correction code, or the like.
Particularly, the case of using a transmission line in which deterioration of the transmission quality is presumed, as in a magnetic recording medium or a communication satellite or the like, it is particularly necessary to pay attention to the countermeasure for such a code error.
FIG. 8 is a block diagram showing a schematic construction of a conventional image transmitting and receiving system.
In the diagram, reference numeral 101 denotes a terminal to which an image signal is supplied. The image signal supplied from the terminal 101 is converted into a digital signal by an analog/digital (hereinafter referred to as an A/D) converter 102. The digital image signal is coded by a high efficiency coding circuit 103 and an information amount (band) is compressed.
The image information compressed by the coding circuit 103 is supplied to an error correction coding circuit 104. A parity check bit to correct the code error is added (the error correction coding is performed) to the image information and, after that, the image signal is sent to a transmission line 105.
On the reception side, a data train transmitted through the transmission line 105 is first stored into a memory 106. The code error correction using the above parity check bit is executed in an error correction unit 107 which accesses the memory 106. The image information which has been subjected to the code error correction is generated from the memory 106 and supplied to a high efficiency decoding circuit 108. The decoding circuit 108 executes a decoding process opposite to that of the high efficiency coding circuit 103. That is, the decoding circuit 108 expands the information amount (band) and returns to the original digital image signal. The digital image signal is converted into an analog signal by a digital/analog (hereinafter referred to as a D/A) converter 109 and is generated as an analog image signal from a terminal 110.
In FIG. 8, various kinds of constructions of the high efficiency coding circuit 103, namely, various kinds of image compressing methods, have been proposed. As a typical one of the color image coding methods, what is called an ADCT method has been proposed. The ADCT method has been described in detail in Takahiro Saito, et al., "The Coding Method of Still Image", the papers of The Institute of Television Engineers of Japan, Vol. 44, No. 2, (1990), Hiroshi Ochi, et al., "The Tendency of International Standard for Coding a Still Image", the papers 14 of the National Conference of The Institute of Image Electrical Engineers of Japan, 1988, and the like.
FIG. 9 is a block diagram schematically showing a construction of the high efficiency coding circuit of an image using the above ADCT method.
It is now assumed that an image signal which is supplied to a terminal 111 in the diagram is a digital data train which has been converted into eight bits, namely, 256 gradations/color through the A/D converter 102 in FIG. 8. It is also assumed that the number of colors is set to three or four colors such as RGB, YUV, YP.sub.b P.sub.r, YMCK, or the like.
The input digital image signal is immediately subjected to a two-dimensional discrete cosine transformation (hereinafter, referred to as a DCT) by a DCT converter 112 on a subblock unit basis of (8.times.8) pixels.
The DCT converted data (hereinafter, referred to as conversion coefficients) of (8.times.8) words is quantized by a linear quantization circuit 113. Quantization step sizes are different every conversion coefficient. That is, the quantization step size for each conversion coefficient is set to a value in which (8.times.8) quantization matrix elements from a quantization matrix generating circuit 114 are multiplied by 2.sup.S times by a multiplier 116.
The quantization matrix elements are determined in consideration of the fact that the visibility of quantization noises differs for every conversion coefficient of (8.times.8) words. Table 1 shows an example of the quantization matrix elements.
TABLE 1 ______________________________________ Example of quantization matrix elements ______________________________________ 16 11 10 16 24 40 51 61 12 12 14 19 26 58 60 55 14 13 16 24 40 57 69 56 14 17 22 29 51 87 80 62 18 22 37 56 68 109 103 77 24 35 55 64 81 104 113 92 49 64 78 87 103 121 120 101 72 92 95 98 112 100 103 99 ______________________________________
The data of 2.sup.S is obtained from a data generator 115. S denotes 0 or a positive or negative integer and is called a scaling factor. A picture quality or a data amount is controlled by the value of S.
The DC component in each of the quantized conversion coefficients, that is, the DC conversion coefficient (hereinafter, referred to as a DC component) in the matrix of (8.times.8) is supplied to a one-dimensional prediction differential circuit 117. A prediction error obtained in the circuit 117 is Huffman coded by a Huffman coding circuit 118. Practically speaking, a quantization output of the prediction error is divided into a plurality of groups. First, the identification number of the group which belongs to the prediction error is Huffman coded and information indicating that the value of the prediction error is the value of that one of the groups is expressed by an equivalent length code.
The conversion coefficient other than the DC component, that is, the AC conversion coefficient (hereinafter referred to as an AC component) is supplied to a zigzag scanning circuit 119. As shown in FIG. 10, the AC conversion coefficients are zigzag scanned by two-dimensional frequencies in the direction from the low frequency component to the high frequency component. A combination of the conversion coefficients (hereinafter referred to as significant coefficients) in which the quantization output is not equal to 0 and the number (run length) of conversion coefficients (hereinafter referred to as insignificant coefficients) in which the quantization output is equal to 0 and which exist between the significant coefficients just before them is generated from the zigzag scanning circuit 119 to a Huffman coding circuit 120.
In the Huffman coding circuit 120, the significant coefficients are classified into a plurality of groups in accordance with the values of the significant coefficients, a combination of the identification number of each group and the run length is Huffman coded, and information indicating that the value of the significant coefficient is a value of that one of the groups is subsequently expressed by an equivalent length code.
Outputs from the Huffman coding circuits 118 and 120 are multiplexed by a multiplexing circuit 121. The multiplexed signal is supplied as a coded output from a terminal 122 to the error correction coding circuit 104 at the post stage.
According to the high efficiency coding as mentioned above, even when the information amount is compressed to a fraction, no deterioration occurs in the image and an extremely high efficiency compression can be executed.
FIG. 11 is a diagram showing a data transmission format according to the conventional system as mentioned above.
In the example shown in the diagram, transmission data has a two-dimensional construction using a double Reed Solomon code which is well known as an error detection and correction code.
A C2 parity of four symbols is added to 124 symbols (hereinafter, one symbol=8 bits) in the lateral direction of the compressed information of the image to be transmitted and four symbols of voice digital information (not shown). A C1 parity of four symbols is also added to 128 symbols in the vertical direction of the image information. Due to this, errors up to two symbols can be corrected with respect to each of the vertical and lateral directions.
On the upper side of FIG. 11, a sync code of two symbols, a transmission ID of two symbols, and control information of two symbols are added to one lateral train, that is, 124 symbols of the image information, four symbols of the voice information, and four symbols of the C2 parity or to 132 symbols of all of the C1 parities. A transmission data block comprising the above symbols as a whole is shown.
One transmitting synchronization (1 ECC block) is constructed by 132 transmission blocks each of which comprises the image information, voice information, C1 parity, C2 parity, sync code, transmission ID, and control information shown on the upper side of FIG. 11. The control information here denotes the foregoing scaling factor, the transmitting method of the voice, the mode information such as stereophonic, bilingual, or the like, the key information for scrambling, the selection information of a receiver, or the like.
However, when the compression of good compression efficiency as mentioned above is executed, that is, when the information is compressed at a high compression ratio and the compressed information is transmitted, the degree of influence which is exerted on the image by one code error is large.
For instance, in the case where the variable length coding as mentioned above has been executed, the subsequent decoding process cannot be performed, so that there is a case where the image after the occurrence of an error fluctuates and enters a state such that it is fairly hard to see.
In the conventional data transmitting format as mentioned above, an amount of image information per unit time changes in dependence on the image. However, an amount of voice information to the image information is always constant. Therefore, the area of the voice information in FIG. 11 must be set so as to have a fairly wide range, so that a long time is expended in vain to transmit the voice information.
Such a problem occurs not only in the voice information but also in the control information. Namely, time is expended in vain to transmit the control information, so that a transmission capacity of the image information relatively decreases and the picture quality is deteriorated.